There are many well-known varieties of feminine hygiene products, baby and adult diapers, tissues, wipes and other implements configured for the absorption of human body fluids. Some of these products, such as sanitary napkins and panty liners, have a pair of wings or flaps that extend laterally from the longitudinal side edges of an absorbent, central portion and are intended to be folded around the edges of the wearer's undergarment during use.
In the manufacturing process of these absorbent articles, a continuous web of layered materials typically is driven through a high-speed machine to form several hundred articles per minute. The web may be formed, for example, from a pair of thin, continuous layers (a backsheet and a topsheet) of a polymer film and/or non-woven web that retain a series of individual, absorbent cores that are spaced evenly apart along the longitudinal axis of the web. The process may include cutting each absorbent article from the continuous web of material, folding the pair of wings around the central portion, applying an adhesive to a release tape, and attaching the release tape to the wings (not necessarily in that order). The wearer removes this release tape to expose the adhesive that has transferred to the backsheet in order to attach the wings to the undergarment.
During the step of folding the pair of wings around the central portion, the article may be conveyed over one or more stationary, contoured rails, which are known in the art as folding plows or folding boards. The folding plows gradually fold each wing 180 degrees from the original plane of the conveyed, web material. Ideally, the wings are consistently folded along fold lines that are parallel to the longitudinal axis of the article so that the free ends of the folded wings come together approximately edge-to-edge.
Unfortunately, sometimes during the folding step, one or both of the wings of an article are folded into a skewed configuration, thereby resulting in a gap between the edges of the free ends of the wings. Generally, manufactured articles having this “skew” also may be excessively wrinkled or may have irregular folds. Similarly, one or both wings may fold-over on itself, either inwardly or outwardly. In either the skew or the fold-over situation, it is possible for some of the adhesive on the release tape to transfer to the topsheet that eventually comes into contact with the wearer's body, causing her considerable discomfort.
Accordingly, there is a need for an improved folding system and process for folding the wings of absorbent articles as they are manufactured from a continuous moving web, whereby wing-folding skew and fold-over are minimized.
Conventional methods for detecting folding process variations in a cost-effective manner have been only partially successful. For example, vision systems have been adapted for use on production lines of absorbent articles to detect product defects such as wings having skewed configurations. However, using the vision systems does not always lead to elimination of the root cause of the problem. Even when skewing is detected, significant production time may be lost in finding the source of the problem and making the necessary repairs and/or design modifications. Traditionally, many types of process and production line component improvements in such manufacturing environments are accomplished using highly iterative, physical testing/prototyping methods, which may be very costly. In addition, it may not always be possible to easily observe or appreciate the effects of the numerous, complex, physical phenomena that may be occuring during the high speed production of absorbent articles. For the folding process described above, such phenomena may include inertial loading on the conveyed articles, changing material properties, aerodynamic forces and friction, especially at interfaces between the conveyed articles and the folding plows.
Accordingly, there is also a need for a method of analyzing the folding system during operation, especially for predicting the magnitude of wing-folding skew, and for optimizing the design of the folding system and process.